The present invention relates to hand held hammer machines comprising a housing with a cylinder therein, in which a reciprocating drive piston via a gas cushion in a working chamber repeatedly drives a hammer piston to impact on and to return from the neck of a tool carried by the machine housing as soon as a feeding force is applied via the machine housing to the tool and spring means interposed therebetween are compressed.
In prior embodiments such machines, particularly if intended for heavy work such as breaking and of which one is described for example in EP publication 388.383 Al, are liable to the hazard of piston collision at application of too strong a feeding force. Such is the case when similarly to what may happen with pneumatic drills or breakers the operator erroneously believes being able to increase the working efficiency by hanging weights on the machine. Another inconvenience is that, although piston collision will be avoided at moderate overfeed, the hammer piston nevertheless will operate under disturbed conditions and at impact will fall in alignment with functionally important porting in the cylinder wall so that the hammer piston seal eventually will be damaged by the edges of said porting and piston leakage and work interruptions will occur. Another inconvenience is that the impact motor of the machine starts to pound as soon as the tool is applied against the surface to be worked upon. That means that the initial collaring or pointing from the very first contact with the working face has to be made under percussive action and, depending on the motor type often under full rotative motor speed, i.e. under full impact power, which makes it difficult to keep the tool exactly on the working spot aimed-at and also exposes the operator to injuries due to recoil and misdirected blows.